Bottom Line:
Unlike vertically transmitted endosymbionts, which have broad effects on their host's germ line, the extracellular gut microbiota is transmitted horizontally and is not known to influence the germ line.We further show that the main impact on oogenesis is linked to the lack of gut Acetobacter species, and we identify the Drosophila Aldehyde dehydrogenase (Aldh) gene as an apparent mediator of repressed oogenesis in Acetobacter-depleted flies.The finding of interactions between the gut microbiota and the germ line has implications for reproduction, developmental robustness and adaptation.

ABSTRACTUnlike vertically transmitted endosymbionts, which have broad effects on their host's germ line, the extracellular gut microbiota is transmitted horizontally and is not known to influence the germ line. Here we provide evidence supporting the influence of these gut bacteria on the germ line of Drosophila melanogaster. Removal of the gut bacteria represses oogenesis, expedites maternal-to-zygotic-transition in the offspring and unmasks hidden phenotypic variation in mutants. We further show that the main impact on oogenesis is linked to the lack of gut Acetobacter species, and we identify the Drosophila Aldehyde dehydrogenase (Aldh) gene as an apparent mediator of repressed oogenesis in Acetobacter-depleted flies. The finding of interactions between the gut microbiota and the germ line has implications for reproduction, developmental robustness and adaptation.

f2: Expedited maternal-to-zygotic-transition and faster development of embryos of bacterial-depleted flies.(a) Two alternative scenarios that can explain global reduction in maternal RNA and reciprocal increase in zygotic transcripts. (b) Representative images of DAPI-stained embryos at 40 min AED (left) and the respective numbers of nuclei per embryo (right), shown for embryos of bacterial-depleted (Dechor. in F1) and control embryos. (c) Transcript fold-change of representative maternal and zygotic genes measured by qPCR in embryos of bacterial-depleted flies at 40 min and 2 h AED. Mean fold-change±s.e. relative to embryos of untreated flies at the respective time. Based on three biological replicates. (d) Effect of bacterial removal by dechorionation on the time of hatching in the next generation. Median time±s.e., based on three replicated experiments, each with >20 embryos. (e) Average duration of nuclear division cycles 11, 12 and 13 in embryos of bacterial-depleted flies and control embryos. Mean±s.e., n≥6 time courses for each case. (f) Density of cells at embryo mid-section immediately following the onset of cellularization. Mean number of cells per 100 μm±s.e., n≥5 time courses. * P<0.05, ** P<0.01 (Student's t-test).

Mentions:
The more progressed stage of development of embryos of bacterial-depleted flies at 2 h AED could reflect initial difference at the time of egg deposition or, alternatively, faster transition to zygotic transcription (Fig. 2a). To determine which of these scenarios is more likely, we analysed embryos at an earlier stage of development. Analysis of 4,6-diamidino-2-phenylindole (DAPI)-stained embryos revealed that the number of nuclei in 40 min AED embryos of bacterial-depleted flies does not exceed the number in embryos of intact flies (Fig. 2b). Lack of initial staging difference was further supported by indistinguishable mRNA levels of representative maternal and zygotic genes at 40 min AED (Fig. 2c). The clear staging difference at 2 h but not at 40 min AED indicated that the embryos of bacterial-depleted flies likely undergo expedited maternal-to-zygotic transition compared with embryos of intact flies. This conclusion was highly consistent with analysis of overall embryonic duration, which showed that the median time of hatching of embryos of bacterial-depleted flies is 3 h shorter compared with control embryos (Fig. 2d; Supplementary Movie 1). The gradual increase in staging (no detectable change at 40 min AED, 30–40 min difference at 2 h AED and 3 h difference at hatching) indicate that embryos of bacterial-depleted flies develop faster than control embryos. To determine how this expedited development is correlated with nuclear divisions in the pre-cellularization embryo, we monitored the division cycles 11–13 by time-lapse confocal microscopy applied to His2Av-mRFP-tagged embryos. Cycle time measurement revealed shorter durations in embryos of flies that were depleted of their gut bacteria (Fig. 2e; Supplementary Movie 2; Supplementary Fig. 5). While providing additional indication for expedited development, the estimated time difference based on cycle durations appeared to be smaller than the estimation by the genome-wide transcript profile. Further analysis of cell density at the onset of cellularization showed that the overall number of nuclear divisions is not affected by the expedited development, as indicated by indistinguishable cell densities (Fig. 2f). Survival to adulthood of embryos of bacterial-depleted flies was also unaffected (Supplementary Fig. 1C), demonstrating ability to adjust the rate of embryonic development in a non-deleterious manner.

f2: Expedited maternal-to-zygotic-transition and faster development of embryos of bacterial-depleted flies.(a) Two alternative scenarios that can explain global reduction in maternal RNA and reciprocal increase in zygotic transcripts. (b) Representative images of DAPI-stained embryos at 40 min AED (left) and the respective numbers of nuclei per embryo (right), shown for embryos of bacterial-depleted (Dechor. in F1) and control embryos. (c) Transcript fold-change of representative maternal and zygotic genes measured by qPCR in embryos of bacterial-depleted flies at 40 min and 2 h AED. Mean fold-change±s.e. relative to embryos of untreated flies at the respective time. Based on three biological replicates. (d) Effect of bacterial removal by dechorionation on the time of hatching in the next generation. Median time±s.e., based on three replicated experiments, each with >20 embryos. (e) Average duration of nuclear division cycles 11, 12 and 13 in embryos of bacterial-depleted flies and control embryos. Mean±s.e., n≥6 time courses for each case. (f) Density of cells at embryo mid-section immediately following the onset of cellularization. Mean number of cells per 100 μm±s.e., n≥5 time courses. * P<0.05, ** P<0.01 (Student's t-test).

Mentions:
The more progressed stage of development of embryos of bacterial-depleted flies at 2 h AED could reflect initial difference at the time of egg deposition or, alternatively, faster transition to zygotic transcription (Fig. 2a). To determine which of these scenarios is more likely, we analysed embryos at an earlier stage of development. Analysis of 4,6-diamidino-2-phenylindole (DAPI)-stained embryos revealed that the number of nuclei in 40 min AED embryos of bacterial-depleted flies does not exceed the number in embryos of intact flies (Fig. 2b). Lack of initial staging difference was further supported by indistinguishable mRNA levels of representative maternal and zygotic genes at 40 min AED (Fig. 2c). The clear staging difference at 2 h but not at 40 min AED indicated that the embryos of bacterial-depleted flies likely undergo expedited maternal-to-zygotic transition compared with embryos of intact flies. This conclusion was highly consistent with analysis of overall embryonic duration, which showed that the median time of hatching of embryos of bacterial-depleted flies is 3 h shorter compared with control embryos (Fig. 2d; Supplementary Movie 1). The gradual increase in staging (no detectable change at 40 min AED, 30–40 min difference at 2 h AED and 3 h difference at hatching) indicate that embryos of bacterial-depleted flies develop faster than control embryos. To determine how this expedited development is correlated with nuclear divisions in the pre-cellularization embryo, we monitored the division cycles 11–13 by time-lapse confocal microscopy applied to His2Av-mRFP-tagged embryos. Cycle time measurement revealed shorter durations in embryos of flies that were depleted of their gut bacteria (Fig. 2e; Supplementary Movie 2; Supplementary Fig. 5). While providing additional indication for expedited development, the estimated time difference based on cycle durations appeared to be smaller than the estimation by the genome-wide transcript profile. Further analysis of cell density at the onset of cellularization showed that the overall number of nuclear divisions is not affected by the expedited development, as indicated by indistinguishable cell densities (Fig. 2f). Survival to adulthood of embryos of bacterial-depleted flies was also unaffected (Supplementary Fig. 1C), demonstrating ability to adjust the rate of embryonic development in a non-deleterious manner.

Bottom Line:
Unlike vertically transmitted endosymbionts, which have broad effects on their host's germ line, the extracellular gut microbiota is transmitted horizontally and is not known to influence the germ line.We further show that the main impact on oogenesis is linked to the lack of gut Acetobacter species, and we identify the Drosophila Aldehyde dehydrogenase (Aldh) gene as an apparent mediator of repressed oogenesis in Acetobacter-depleted flies.The finding of interactions between the gut microbiota and the germ line has implications for reproduction, developmental robustness and adaptation.

ABSTRACTUnlike vertically transmitted endosymbionts, which have broad effects on their host's germ line, the extracellular gut microbiota is transmitted horizontally and is not known to influence the germ line. Here we provide evidence supporting the influence of these gut bacteria on the germ line of Drosophila melanogaster. Removal of the gut bacteria represses oogenesis, expedites maternal-to-zygotic-transition in the offspring and unmasks hidden phenotypic variation in mutants. We further show that the main impact on oogenesis is linked to the lack of gut Acetobacter species, and we identify the Drosophila Aldehyde dehydrogenase (Aldh) gene as an apparent mediator of repressed oogenesis in Acetobacter-depleted flies. The finding of interactions between the gut microbiota and the germ line has implications for reproduction, developmental robustness and adaptation.